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Full text of "Experiments on the Refrangibility of the Invisible Rays of the Sun. By William Herschel, LL. D. F. R. S."

C ^84, 3 



XIV. Experiments on the Refrangibility of the invisible Rays of the 
Sun. By William Herschel, LUD. F. R. S. 



Read April 2^4, 1800. 

In that section of my former paper which treats of radiant heat, 
it was hinted, though from imperfect experiments, that the range 
of its refrangibility is probably more extensive than that of the 
prismatic colours ; but, having lately had some favourable sun- 
shine, and obtained a sufficient confirmation of the same, it will 
be proper to add the following experiments to those which have 
been given. 

I provided a small stand, with four short legs, and covered it 
with white paper.* On this I drew five lines, parallel to one end 
of the stand, at half an inch distance from each other, but so 
that the first of the lines might only be j; of an inch from the 
edge. These lines I intersected at right angles with three others ; 
the 2d and 3d whereof were, respectively, at 2|- and at 4 inches 
from the first. 

The same thermometer^ that have before been marked 
No. 1, s, and 3, mounted upon their small inclined planes, were 
then placed so as to have the centres of the shadow of their 
balls thrown on the intersection of these lines. Now, setting 
my little stand upon a table, I caused the prismatic spectrum to 
fall with its extreme colour upon the edge of the paper, so that 
none might advance beyond the first line. In this arrangement, 

♦ See Plate XL 



Dr. Herscwei's Experiments, &c, 285 

all the spectrum, except the vanishing last quarter of an inch, 
which served as a direction, passed down by the edge of the 
stand, and could not interfere with the experiments. I had also 
now used the precaution of darkening the window in which the 
prism was placed, by fixing up a thick dark green curtain, to 
keep out as much light as convenient. 

The thermometers being perfectly settled at the temperature 
of the room, I placed the stand so that part of the red colour, 
refracted by the prism, fell on the edge of the paper, before the 
thermometer No. 1, and about half way, or i-^ inch, towards 
the second : it consequently did not come before that, or the 
3d thermometer, both which were to be my standards. During 
the experiment, I kept the last termination of visible red 
carefully to the first line, as a limit assigned to it, by gently 
moving the stand when required ; and found the thermometers, 
which were all placed on the second linp, affected as follows. 

No. 3. 

- 44 

Here the thermometer No. 1 rose 6| degrees, in 10 minutes^ 
when its centre was placed ^ inch beyond visible light. 

In order to have a confirmation of this fact, I cooled the 
thermometer No. 1, and placed No. s in the room of it : I also 
put No. 3 in the plaee of No. 2, and No. 1 in that of No. 3 ; 
and, having exposed them as before, arraja^ed on the second 
line, I had the following result. 



No. 1. 






No. s 


45 


M» 


- 


45 


49 


- 


- 


45 


51 


- 


- 


44| 


50i 


- 


- 


43i 



386 Dr. Herschel's Experiments on the Refrangihility 



No. 3. 


No. 3. 


No. 1. 


44 


44 


45 


47 


- 44 


45 


^H - 


44 


45 


4^1 - 


44 


45 



Here the thermometer No. 2 rose 2^ degrees, in 13 minutes; 
and being, as has been noticed before, much more sensible than 
No. 1, it came to the temperature of its situation in a short time; 
but I left it exposed longer, on purpose to be perfectly assured 
of the result. Its shewing but 2| degrees advance, when No. 1 
shewed 6|-, has also been accounted for before. 

It being now evident that there was a refraction of rays coming 
from the sun, which, though not fit for vision, were yet highly 
invested with a power of occasioning heat, I proceeded to ex- 
amine its extent as follows. 

The thermometers were arranged on the third line, instead 
of the second; and the stand was, as before, immersed up to the 
first, in the coloured margin of the vanishing red rays. The 
result was thus- 

No. 1. No. 2. No. 3. 



46 


46 


- 


- 45| 


50 


- 46^ 


- 


- 46 


5ii - 


- 4^1 


- 


- 46i 


5^i -- 


47 


- 


- . 46I 



Here the thermometer No. 1 rose 5^ degrees, in 13 minutes^ 
at 1 inch behind the visible light of the red rays. 

I placed now the thermometers on the 4th line, instead of 
the 3d ; and, proceeding as before, I had the following result. 



of the invisible Rays of the Sun. £87 

No. 1. No. 2. No. 3. 

48i - - 48i - - 471 
5ii - - 48I - - 471- 

Therefore, the thermometer No. 1 rose 3- degree, in 10 
minutes, at 1^ inch beyond the visible light of the red rays. 

I might now have gone on to the 5th line ; but, so fine a day, 
with regard to clearness of sky and perfect calmness, was not 
to be expected often, at this time of the year; I therefore 
hastened to make a trial of the other extreme of the prismatic 
spectrum. This was attended with some difficulty, as the illu- 
mination of the violet rays is so feeble, that a precise termination 
of it cannot be perceived. However, as well as could be judged, 
I placed the thermometers one inch beyond the reach of the 
violet rays, and found the ^result as follows. 

►.1* No. 2. No. 3. 



48 




- 


48 


-' 


471 


48 


' mm 


- 


48 


- 


47i 


48 


- 


- 


47i 


mm mm 


47 


m 


- 


*■ 


471 


- 


47 


48 


- 


- 


48 


- 


471 



Here the several indications of the thermometers, two of 
which. No. 1 and 2, were used as variable, while the 3d was 
kept as the standard, were read off during a time that lasted 
13 minutes ; but they aflbrd, as may be seen by inspection, no 
ground for ascribing any of their small changes to other causes 
than the accidental disturbance which will arise from the motion 
of the air, in a room where some employment is carried on. 

I exposed the thermometer now to the line of the very first 
perceptible violet light; but so that No. 1 and > might again 

MDCCC, P p 



£88 Dr:lrLERscuEL's Experiments on the Refrangibility 

be in the illumination, while No, 3 remained a standard. The 
result proved as follows. 

No. 1. No. 2, No. 3. 



48 


- 


» 


48 


- 


- 


47i 


48i 


- 


- 


48 


- 


- 


471 


48i 


- 


- 


m 


- 


- 


471 


49 


mm 


- 


48i 


- 


- 


471 



Here the thermometer No. 1 rose 1 degree, in 15 minutes; 
and No. 2 rose |- degree, in the same time. 

From these last experiments, I was now sufficiently persuaded, 
that no rays which might fall beyond the violet, could have 
any perceptible power, either of illuminating or of heating ; and 
that both these powers continued together throughout the pris- 
matic spectrum, and ended where the faintest violet vanishes. 

A very material point remained still to be determined, which 
was, the situation of the maximum of the heating power. 

As I knew already that it did not lie on the violet side of the 
red, I began at the full red colour, and exposed my thermometers, 
arranged on a line, so as to have the ball of No, 1 in the midst 
of its rays, while the other two remained at the side, unaffected 
by them. 



so. 1. 




No. 2. 




No. 


4>H - 


- 


48^ 


- 


48 


55i 


- 


48i 


- 


48 


55i 


- 


481- 


- 


48 



Here the thermometer No. i rose 7 degrees, in 10 minutes, by 
an exposure to the full red coloured rays. 

I drew back the stand, till the centre of the ball of No. 1 was 



of the invisible Rays of the Sun. 28^ 

just at the vanishing of the red colour, so that half its ball was 
within, and half without, the visible rays of the sun. 

No. !• No. 2. No. 3* 

481. - - 481 - - 48 

55i - - 481- - - 48 
57 - - 49 - - 48i 

Here the thermometer No. 1 rose 8 degrees, in 10 minutes. 
By way of not losing time, in order to connect these last 
observations the better together, I did not bring back the ther- 
mometer No. 1 to the temperature of the room, being already 
well acquainted with its rate of shewing, compared to that of 
No. 2, but went on to the next experiment, by withdrawing the 
stand, till the whole ball of No. 1 was completely out of the sun's 
visible rays, yet so as to bring the termination of the line of the 
red colour as near the outside of the ball as could be, without 
touching it 



^0. 1. 






No. 3. 




No. 3. 


57 


- 


M* 


49' 


- 


48^ 


5H 


- 


- 


49i 


- 


49 


59 


- 


- 


5oi 


- 


49i 


59 


- 


- 


50 


- 


49t 



Here the thermometer No. 1 rose, in 10 minutes, another 
degree higher than in its former situation it could be brought 
up to ; and was now 9 degrees above the standard. The ball 
of this thermometer, as has been noticed, is exactly half an inch 
in diameter ; and its centre therefore was ^ inch beyond the 
visible illumination, to which no part of it was exposed. 

It would not have been proper to compare these last obser- 
vations with those taken at an earlier period this morning, in 

Pps 



L 

2 



^90 Dr. llERSCEmfs Experiments on the Ref Tangibility 

order to obtain a true maximum, as the sun was now more 

powerful than it had been at that time; for which reason, I 

caused the hne of termination of visible light, now to fall again 

just i inch from the centre of the ball; and had the following 

result. 

No. 1. No. 2. No. g: 

571 - - 50 — - 49f 

^i - - 50 - - 49i 

5H - - 50 - - 49- 

And here the thermometer No. 1 rose, in 16 minutes, 8| 

degrees, when its centre was |- inch out of the visible rays of 

the sun. Now, as before we had a rising of 9 degrees, and 

here 8|^, the difference is almost too trifling to suppose, that 

this latter situation of the thermometer was much beyond the 

maximum of the heating power; while, at the same time, the 

experiment sufficiently indicates, that the place inquired after 

V 

need not be looked for at a greater distance. 

It will now be easy to draw the result of these observations 
into a very narrow compass. 

The first four experiments prove, that there are rays coming 
from the sun, which are less refrangible than any of those that 
affect the sight. They are invested with a high power of heating 
bodies, but with none of illuminating objects ; and this explains 
the reason why they have hitherto escaped unnoticed. 

My present intention is, not to assign the angle of the least 
refrangibility belonging to these rays, for which purpose more 
accurate, repeated, and extended experiments are required. But, 
at the distance of 52 inches from the prism> there was still a 
considerable heating power exerted by our invisible rays, one 



of the invisible Kays of the Sun. %gt 

inch and a half beyond the red ones, measured upon their pro- 
jeetion on: a horizontal plane. I have no doubt but that their 
efficaGyma^r be traced still somewhat farther. 

The 5th and 6th experiments shew, that the power of heating 
is extended to the utmost limits of the visible violet rays, but 
not bey ond them ; and that it is gradually impaired, as the rays 
grow more refrangible. \ 

The four last experiments prove, that the maximum of the 
heating power is vested among the invisible rays ; and is pro- 
bably not less than half an inch beyond the last visible ones, 
when projected in the manner before mentioned. The same 
experiments also shew, that the sun's invisible rays, in their less 
refr^igible state, and considerably beyond the maximum, stilt 
exert a heating power fuliy equal to that of red^coloured light; 
and that^ consequently, if we may infer the quantity of tihe effi'- 
cient ftenii the efesti produjced,: the invisible rays of the sun 
p^olmbly far exceedr the visible ones in number. 

To concMde, if we calli %ii^ those rays wWch illuminate 
objects, and radiant heat, those whicli fieat bodies, it may be 
inquired, whether light be essentially different from radiant 
heat ? In answer to which I would suggest, that w^e are not 
allowed, by the rules of philosophizing, to admit two different 
causes to explain certain effects, if they may be accounted for 
by one. A beam of radiant heat, emanating from the sun, 
consists of rays that ate differently refrangible. The range of 
their extent, when dispersed by a prism, begins at violtet- 
coloured light, where they are most refracted^ and have the 
least eflScacy. We have traced these calorifie rays throughout 
the whole extent of the prismatic spectrum; and found their 
power increasing, while tibeir r^rangibility was^ lessened^ as far 



^^2 Dr. Her'schel*s Experiments^ &c. 

as to the confines of red-coloured light. But their diminishing 
refrangibility, and increasing power, did not stop here ; for we 
have pursued them a considerable way beyond the^rtjmaif/f: 
spectrum^ into an invisible state, still exerting their increasing 
energy, with a decrease of refrangibihty up to the maximum of 
their power; and have also traced them to that state where, 
though still less refracted, their energy, on account, we may 
suppose, of their now faihng density, decreased pretty fast; 
after which, the invisible thermometrical spectrum, ii I may so 
call it, soon vanished. 

If this be a true account of solar heat, for the support of 
which I appeal to my experiments, it remains only for us to 
admit, that such of the rays of the sun as have the refrangibility 
of those which are contained in the prismatic spectrum, by the 
construction of the organs of sight, are admitted, under the 
appearance of light and colours ; and that the rest, being stopped 
in the coats and humours of the eye, act upon them, as they 
are known to do upon all the other parts of our body, by occa- 
sioning a sensation of heat* 

Slough, near Windsor, 
March 17, 1800^ 

Explanation or Plate XL 

IN WHICH IS GIVEN A VIEW OF THE APPARATUS. 

A B. The small stand. 
1, 2, 3. The thermometers upon it. 
C D. The prism at the window. 

E. The spectrum thrown upon the table, so as to bring the 
last quarter of an inch of the red colour upon the stand. 



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